U.S. patent number 5,893,358 [Application Number 08/964,087] was granted by the patent office on 1999-04-13 for pellet fuel burner for heating and drying systems.
This patent grant is currently assigned to Pyro Industries, Inc.. Invention is credited to Oliver J. Whitfield.
United States Patent |
5,893,358 |
Whitfield |
April 13, 1999 |
Pellet fuel burner for heating and drying systems
Abstract
A pellet fuel burner includes a compact thermally insulated
firebox for providing hot gases to a boiler, furnace or dryer. The
apparatus includes a grate that includes a reciprocating arm for
removing clinkers and ash from the grate. Pellet fuel feed is
provided by a hopper that is isolated from the firebox by an
airlock. A combustion air manifold is provided around the firebox
to preheat primary and secondary combustion air.
Inventors: |
Whitfield; Oliver J. (Bow,
WA) |
Assignee: |
Pyro Industries, Inc.
(Burlington, WA)
|
Family
ID: |
25508111 |
Appl.
No.: |
08/964,087 |
Filed: |
November 4, 1997 |
Current U.S.
Class: |
126/73; 110/110;
126/68; 126/112; 110/297; 110/233 |
Current CPC
Class: |
F23H
15/00 (20130101); F23M 5/00 (20130101); F23L
15/00 (20130101); F23B 5/04 (20130101); F23L
5/02 (20130101); F23K 3/16 (20130101); F23B
1/38 (20130101); F23G 2209/261 (20130101); Y02E
20/34 (20130101); F23M 2900/05004 (20130101); F23K
2900/03001 (20130101); F23K 2203/202 (20130101) |
Current International
Class: |
F23K
3/16 (20060101); F23L 5/02 (20060101); F23M
5/00 (20060101); F23K 3/00 (20060101); F23L
5/00 (20060101); F23H 15/00 (20060101); F23L
15/00 (20060101); F24B 013/04 () |
Field of
Search: |
;126/73,68,76,77,151,146,112 ;110/297,233,110,300,298 ;432/121 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yeung; James C.
Attorney, Agent or Firm: Christensen O'Connor Johnson &
Kindness PLLC
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An apparatus fueled by pelletized fuel for producing thermal
energy in the form of hot gas comprising:
a firebox comprising a shell of refractory material having an
inlet, an outlet, a baffle between the inlet and the outlet, an
exterior surface and an interior surface, the shell and the baffle
cooperating to define an internal gas pathway;
a thermal shell comprising a thermal insulating material contacting
the exterior surface and substantially surrounding the firebox;
a grate located adjacent the inlet where pelletized fuel is
combusted;
a fuel feed conduit for delivering the pelletized fuel to the
inlet; and
a combustion air manifold surrounding the thermal shell for
delivering combustion air from a combustion air source around the
thermal shell to the grate, the combustion air being heated as it
passes from the combustion air source to the grate.
2. The apparatus of claim 1, further comprising a source of
pelletized wood fuel and an airlock between the source of the
pelletized wood fuel and the firebox.
3. The apparatus of claim 1, wherein the internal gas pathway is
serpentine in shape.
4. The apparatus of claim 1, wherein the hot gas exits the outlet
of the firebox at a temperature of at least 1000.degree. F.
5. The apparatus of claim 4, wherein the hot gas exits the outlet
of the firebox at a temperature of at least 2000.degree. F.
6. The apparatus of claim 1, wherein the combustion air source is a
blower.
7. The apparatus of claim 1, wherein the grate comprises a
plurality of spaced apart rods.
8. The apparatus of claim 7, wherein the grate further comprises a
plurality of reciprocating elements extending between the rods for
removing clinkers and ash from the grate.
9. The apparatus of claim 1, wherein the refractory material is
different from the thermal insulating material.
10. The apparatus of claim 1, wherein the refractory material
comprises a ceramic and the thermal insulating material comprises a
ceramic fiber board.
11. The apparatus of claim 1, wherein the specific thermal energy
output of the apparatus is at least 400K BTU/hr/ft.sup.3.
12. The apparatus of claim 1, wherein the specific thermal energy
output of the apparatus is at least 550K BTU/hr/ft.sup.3.
Description
FIELD OF THE INVENTION
The present invention relates to a pellet fuel burner for providing
hot gas to furnaces or to boilers used for residential and
industrial space heating and other applications.
BACKGROUND OF THE INVENTION
Hydronic space heating systems comprise a boiler that delivers hot
water throughout an area to be heated. Current applications include
wall mounted radiators or tubes embedded in the floors of
residences or industrial sites for distributing the hot water
throughout the home or building. A substantial portion of boilers
for hydronic heating systems are oil or gas fired, using an
external burner that generates hot gases and delivers the hot gases
to a boiler to elevate the temperature of the water. Such oil and
gas burners have been developed to a high level of sophistication
to provide safe, reliable, and efficient burning of the fuel, with
extremely low levels of emissions.
Combustion of fossil fuels such as coal, oil and natural gas
generates substantial quantities of carbon dioxide. This
contributes to the environmental problem of "global warming" that
results from the excessive build-up of these "greenhouse gases."
Wood is commonly used as a residential heating fuel in areas where
wood is readily available and economical. Use of wood has some
obvious drawbacks due to the need for frequent manual loading of
the fuel, storage of the fuel, variability in the moisture of the
fuel and the need to remove ashes resulting from the combustion of
the fuel.
Wood pellets, a dry, processed, wood fuel made from waste sawdust,
chips, etc., can be an economical and environmentally safe fuel
substitute for fossil fuels for the purpose of residential and
industrial space heating. In contrast to fossil fuels, which
contribute to the buildup of greenhouse gases, wood is considered a
"renewable fuel" that contributes no net buildup of atmospheric
carbon dioxide.
The use of these wood pellets in heating stoves is widespread. One
example of such type of stove is described in U.S. Pat. No.
5,488,943, by the applicant of the subject application. In these
types of wood stoves, which are generally free standing or
fireplace inserts, the wood pellets are combusted to provide hot
gas which is passed through a heat exchanger to transfer thermal
energy to room air that occupies the opposite side of the heat
exchanger. Users of these types of pellet fuel heating stoves find
them desirable because of the aesthetic advantages of a natural
flame that is readily visible through the door of the stove.
The attractiveness of wood as an alternative to fossil fuel has led
to the development of several hydronic heating systems that employ
boilers that are fueled by wood pellets. These systems provide an
environmentally friendly alternative to existing fossil fueled
boilers. In those instances where the boilers themselves do not
need to be replaced, the user must decide whether his or her desire
for a burner fueled by a renewable fuel is outweighed by the cost
of replacing an otherwise satisfactory fossil fuel burner. Given
the large number of fossil fuel boilers in use, it would be
desirable if a hot gas generator fueled by wood pellets was
available which could be used to replace the fossil fuel burner of
existing boilers. This would avoid the cost of replacing the entire
boiler unit and would result in less waste of valuable resources.
There have been attempts to design pellet fuel burners that would
replace fossil fuel burners in existing boilers; however, such
designs generally attempt to place the actual burner grate within
the boiler. This approach suffers from the drawback that a specific
burner design is required for each unique boiler design.
In hydronic heating systems, the heat transfer efficiency from the
hot gases produced by the combustion of fuel to the boiler
decreases proportionately to the quantity of air used in the
combustion process. Oil burners typically utilize 15%-30% excess
air above theoretical "stoichiometric" requirements and generate
emissions of carbon monoxide and nitrogen dioxide on the order of
50 parts per million. First generation designs of boilers using
wood pellets as a fuel use 200% excess air or higher and can emit
carbon monoxide levels in excess of 500 parts per million.
With the concern for "global warming" and the push to implement
alternative fuel sources which are renewable and environmentally
friendly, prior designs of burners using wood pellets as a source
of hot gases for hydronic heating systems have met limited
acceptance because of the high levels of excess air required, lower
efficiencies, high levels of carbon monoxide they emit, and the
need for numerous designs to correspond with the multitude of
boiler designs, even though they employ a renewable fuel
source.
SUMMARY OF THE INVENTION
The present invention provides a burner system for automatically
feeding and combusting wood pellets to generate hot gases for use
in a number of applications, such as a boiler or forced air furnace
for residential and industrial space heating. Another application
includes a dryer for agricultural or industrial uses. The burner of
the present invention utilizes quantities of combustion air and
produces quantities of emissions comparable to state-of-the-art oil
or gas burners. The control and automatic features of the burner
permit wood, a "renewable fuel," to be used as a practical,
economical and environmentally safe alternative to oil or gas in
space heating and drying applications. The present invention
achieves these results by using a unique double wall, thermally
insulated combustion chamber and a grate design that promotes
intimate mixing of combustion air with the wood fuel while
processing the inorganic ash downward through the grate. The high
temperatures and effective combustion air mixing achieved with the
combustion chamber of the present invention provide favorable
combustion conditions, resulting in high heat transfer efficiencies
to boilers, furnaces and dryers, and low emissions.
The burner system is designed as an external, stand-alone unit
which delivers hot gases to a boiler or forced air furnace.
Providing the burner independently, the unit can be readily
retrofitted to existing burners and furnaces which currently use
oil or gas fuel. The external, stand-alone nature of the burner
system of the present invention makes the system particularly well
suited for replacing any externally mounted fossil fuel burner that
delivers hot gases to a boiler of furnace. In those instances where
a preexisting oil or gas fueled burner is not external, i.e., is
located within the boiler or furnace, the above described retrofit
involves removal of the oil or gas fuel burner and may involve the
provision of additional hardware to connect the outlet of the
pellet fuel burner to the boiler or furnace so that it can deliver
hot gases thereto.
In accordance with the present invention, a burner fueled by
pelletized fuel for providing thermal energy, in the form of hot
gas, includes a source of pelletized fuel. The source of pelletized
fuel communicates with a combustion chamber or firebox that
includes a shell made of a refractory material, an inlet for
combustion air, an outlet for the combustion gases, a baffle
between the inlet and the outlet, an exterior surface and an
interior surface. The shell and the baffle cooperate to define an
internal gas pathway, preferably serpentine in shape, leading from
the inlet to the outlet. A thermal insulating shell comprising a
thermal insulating material contacts the exterior surface of the
combustion chamber and substantially surrounds the combustion
chamber. A grate is located below the inlet of the combustion
chamber where the pelletized fuel from the fuel source is received
and combusted. The apparatus further includes a combustion air
manifold surrounding the thermal insulating shell for delivering
combustion air from a combustion air source around the thermal
insulating shell to the grate. Preferably, the combustion air is
preheated as it passes from the combustion air source to the grate
by thermal energy that emanates from the combustion chamber and
passes through the thermal insulating shell.
The apparatus of the present invention produces hot combustion
gases at the outlet that are at a temperature of at least
1000.degree. F., and preferably at least 2000.degree. F. The gases
can be delivered to a boiler or furnace for utilization. The
compact nature, thermal insulation of the combustion chamber, and
efficient combustion capabilities combine to provide an apparatus
capable of producing a specific thermal energy output on the order
of 400K BTU/hr/ft.sup.3, and preferably at least 500K
BTU/hr/ft.sup.3.
In a preferred embodiment, the grate comprises a plurality of
spaced apart rods and a reciprocating element that extends between
the rods for agitating the fuel bed and removing clinkers and ash
from the grate. The removed clinkers and ash are collected in an
ash pan positioned below the grate. This ash pan is readily
accessible so that the removal of clinkers and ash can be
accomplished safely and efficiently, using a manual or automatic
auger system.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same becomes
better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings, wherein:
FIG. 1 is a perspective view with a portion cut away of a pellet
fuel burner formed in accordance with the present invention;
FIG. 2 is an exploded view of the components inside the burner
casing;
FIG. 3 is a perspective view of the pellet fuel feed system of the
present invention;
FIG. 4 is a cross-sectional view taken along line 4--4 in FIG. 5
with a portion cut away; and
FIG. 5 is a cross-sectional view taken along line 5--5 in FIG.
4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following description of a pellet burner formed in
accordance with the present invention, the materials for the
individual elements are generally carbon steel, unless otherwise
expressly indicated. The advantages of the present invention can be
achieved by using comparable material and accordingly, the present
invention is not limited to those specific materials set forth
herein.
As used herein, the term "combustion air" refers to gases necessary
to support the combustion of a pelletized fuel. Preferably, the
combustion gas is ambient air.
The following description describes the present invention in the
context of a burner for supplying hot gases to a boiler. It should
be understood that other devices such as a furnace or dryer could
be used in conjunction with the present invention.
Referring to FIGS. 1 and 5, a pellet burner 10 formed in accordance
with the present invention is enclosed within a housing 12. The
illustrated embodiment is a pellet burner for supplying hot gases
to a boiler that supplies hot water to a residential hydronic
heating system. It should be understood that the following
description and principles are equally applicable to larger pellet
burners that would be suitable for use in industrial space heating
applications, drying applications and other thermal applications
that require hot gases, although the specific dimensions may be
proportionately larger or smaller. Housing 12 is a generally
cube-shaped enclosure having front wall 14, opposing rear wall 16,
right sidewall 18, left sidewall 20, top 22 and floor 24. Top 22
includes an orifice through which fuel conduit 26 passes and is
attached to trough 126 as described below in more detail. The
opposite end of fuel conduit 26 is connected to a source of
pelletized fuel such as a hopper (not shown). Passing through the
lower half of front wall 14 is one end of combustion gas conduit
28. The opposite end of combustion gas conduit 28 is configured for
attachment to a device capable of utilizing the hot gases, such as
a boiler or furnace (not shown). The bottom front corner of right
sidewall 18 includes access door 30 which closes an opening that
permits access to the interior of housing 12. As described below in
more detail, ash can be removed from the pellet burner 10 through
door 30. Right sidewall 18 also includes a viewing port 32 to allow
for the visual inspection of the interior of the combustion chamber
described below in more detail.
Housing 12 provides a framework to which other elements of the
pellet burner are attached and supported as described below in more
detail. In addition, housing 12 serves as a safety barrier to
isolate moving parts and hot parts from the surrounding
environment.
Referring specifically to FIGS. 2, 4 and 5, centered between the
right sidewall 18 and left sidewall 20, towards the front of
housing 12, is cube-shaped burner casing 34 that includes left
sidewall 36, opposing right sidewall 38, front sidewall 40 and rear
sidewall 42. Left sidewall 36 and right sidewall 38 extend upward
from floor 24 about two-thirds of the height of housing 12. Rear
wall 42 is spaced rearward from front wall 14 of housing 12 about
two-thirds the depth of housing 12 and extends upward from floor 24
to the same height as left sidewall 36 and right sidewall 38. Front
wall 40 of burner casing 34 is separated from the inner surface of
front wall 14 of housing 12 by a layer of insulation 41. Front wall
40 extends from a location spaced above floor 24 of housing 12 to
the same height as the left sidewall, right sidewall and rear
sidewall of burner casing 34. The upper ends of left sidewall 36,
right sidewall 38, front sidewall 40 and rear sidewall 42 are
connected to each other by top 44 to form enclosed burner casing
34.
Suspended from front sidewall 40 and rear sidewall 42 within burner
casing 34 is firebox support 46. Firebox support 46 is a square
tray having upward extending peripheral sidewalls 48. Extending
forward and rearward of the four corners of firebox support 46 are
support flanges 50. The ends of the support flanges 50 extending
from the front of firebox support 46 are attached to the inner
surface of front sidewall 40. The free ends of support flanges 50
extending from the rear of firebox support 46 are attached to the
inner surface of rear sidewall 42. This attachment can be achieved
by conventional means such as welding, bolting, or riveting the
flanges in place. As a result of securing support flanges to the
burner casing 34, firebox support 46 is suspended above floor 24 of
housing 12. Firebox support 46 is positioned far enough above floor
24 so as to provide ample space for positioning burner grate 52 and
ash pan 54 between the underside of firebox support 56 and floor
24.
Seated on the upper surface of firebox support 46 is firebox 56,
thermal shell 70 and firebox casing 80.
In the illustrated embodiment, firebox 56 is cube shaped having
four vertical sidewalls 62, a floor 64 and a top 66. The walls,
floor and top of firebox 56 are manufactured from a refractory
material capable of withstanding temperatures on the order of at
least 2000.degree. F. and chemical attack by fly ash, possibly in
molten form. A suitable refractory material includes a ceramic
material having silicon carbide as a majority component. Other
refractory materials providing the same desirable properties as a
ceramic material are suitable. The sidewalls 62 and floor 64 are
connected by conventional means to provide an integral box open at
the top. Firebox 56 includes a vertical baffle 68 that extends
horizontally between the right and left sidewalls of firebox 56 and
extends upward from floor 64 of the firebox to a point below the
underside of top 66 when it is secured to the top of vertical
sidewalls 62. Baffle 68 is preferably manufactured from the same
materials as the sidewalls, floor and top of the firebox. The
thicknesses of the walls, floor and top of the firebox are selected
so that the temperature gradient in the firebox walls, floor and
top is low, so as to avoid degradation due to thermal shock
resulting from rapid changes in the temperature of the combustion
gases in the firebox. In the preferred embodiment described herein,
the thickness of the walls, floor and top of the firebox is on the
order of about one inch; however, it should be understood that the
ultimate thickness of the firebox walls will depend upon a number
of factors, including the size of the firebox and the thermal
output of the firebox.
In a preferred embodiment, the volume defined by the internal
chamber of the firebox should be large enough that the residence
time of the gases resulting from the combustion of the pelletized
fuel is long enough to permit complete combustion and minimize the
carbon monoxide and unhealthy hydrocarbon gases exhausted from the
firebox, yet small enough that the heat loss from the firebox is
low, such that the temperature of the gases within the firebox can
be maintained at levels necessary to promote complete combustion.
In the preferred embodiment illustrated in this application, the
internal chamber of the firebox has a specific volume on the order
of 0.1 to 0.3 ft.sup.3. It should be understood that the present
invention is not limited to these specific volumes, and the volume
thereof may be larger or smaller than that described above.
Baffle 68 helps to create turbulence in the gases produced by the
combustion of the pelletized fuel. The turbulence serves to enhance
the mixing of the combustion air and hydrocarbon gases, thus
promoting further combustion. The distance between the top of
baffle 68 and the underside of top 66 is selected so that the
passage of the combustion gases is not impeded. While the firebox
has been illustrated in the context of a specific shape, it should
be understood that other shapes that provide the desirable
characteristics of the firebox as described herein would be
suitable.
Firebox 56 is encased by a thermal insulating shell 70 which is
congruent in its peripheral configuration to firebox 56 and is
sized to snugly receive and surround firebox 56. Thermal insulating
shell 70 does not include an internal baffle. Vertical sidewalls
72, floor 74 and top 76 of thermal shell 70 serve to assist in the
retention of heat within firebox 56 and therefore are manufactured
from a thermally insulating material having high thermal insulating
properties and capable of withstanding temperatures associated with
the exterior surface of the firebox. An example of suitable
thermally insulating materials includes ceramic fiberboard. The
thickness of the walls, floor and top of the thermal insulating
shell are selected so the desired thermal insulation is provided to
maintain the desired temperature within the firebox.
Surrounding the vertical sidewalls 72 of thermal insulating shell
70 is steel firebox frame 80. Firebox frame 80 includes vertical
sidewalls 78 that serve to define a substantially square casing
into which thermal shell 70 is snugly received. In its assembled
form, firebox 56 is positioned within thermal insulating shell 70
which is positioned within firebox frame 80. The external
dimensions of the bottom of firebox frame 80 are such that it fits
snugly within upstanding sidewalls 48 of firebox support 46.
In order to ensure that top 76 of thermal shell 70 is maintained
snugly in place, two S-shaped retaining clips 82 are provided on
the inner surface of top 44 of burner casing 34. The ends of
flanges 82 that are not attached to the underside of top 44 bear
down upon the upper surface of top 76 and serve to hold the top
securely in place.
Referring to FIGS. 2, 4 and 5, floor 45 of firebox support 46,
floor 74 of thermal insulating shell 70 and floor 64 of firebox 56
include a rectangular aperture rearward of baffle 68 that defines
an inlet to firebox 56. Attached to the underside of firebox
support 46 adjacent this inlet is burn grate 52. Burn grate 52 is a
rectangular element having left sidewall 84, an opposing right
sidewall 86 that are connected by a front sidewall 88 and an
opposing rear sidewall 90. The combination of these sidewalls
define a rectangular chamber open at the top and open at the
bottom. Extending between front sidewall 88 and rear sidewall 90
are a plurality of rods 92 whose terminal ends are supported in
holes provided in the front sidewall 88 and rear sidewall 90. Rods
92 are spaced apart a distance that permits the combination of the
rods to support pelletized fuel for combustion. The rod spacing is
selected so that unburned pelletized fuel will not fall between the
rods while ash, which is formed by the combustion, can fall between
the rods. In addition, the rod spacing is selected so that there is
sufficient open space to permit combustion air to pass between the
rods to support combustion of the pellet fuel above the grate. In
the illustrated embodiment, left sidewall 84 includes an orifice
for receiving an igniter 94 for fuel residing on rods 92. One
example of a suitable igniter is a hot gas generator. Other types
of igniters such as resistive heaters and other articles capable of
providing high temperatures to ignite the pellets may be used.
Extending down from left sidewall 84 and right sidewall 86 are
opposing horizontal tracks 96. Tracks 96 include a horizontal
groove that extends forward and rearward. The horizontal groove is
sized to receive guides on the left and right sides of a
reciprocating rake 98 positioned below rods 92. Reciprocating rake
98 comprises a plurality of upstanding fingers that extend through
the spaces between rods 92. The upward extending fingers are in the
shape of inverted T's. In the illustrated embodiment, the
reciprocating rake comprises nine upward extending fingers. The
leftmost and rightmost fingers carry horizontal outward extending
guides that are received by tracks 96 to support the reciprocating
rake beneath grate 52. In order to minimize buildup of ash in the
corners of the grate 52, the second upward extending fingers from
the left edge and the right edge are longer and extend upward
further than the balance of the fingers. Reciprocating rake 98 is
attached by horizontal rod 100 to a motor driven cam assembly 102
that reciprocates the rake forward to rearward at selected
intervals.
Left sidewall 84, right sidewall 86, front sidewall 88 and opposing
rear sidewall 90 of the burn grate may optionally include
additional openings for permitting secondary combustion air to mix
with the fuel. This secondary combustion air is provided via
combustion air manifold 142 described below in more detail.
The burner of the present invention has been described above with
reference to a particular burn grate. It should be understood that
other configurations of a burn grate may be used in the context of
the present invention. For example, other burn grate designs are
described in U.S. Pat. Nos. 5,488,943, 5,383,446, and
5,295,474.
Pellet fuel is provided to the upper surface of burn grate 52
through openings 104 in firebox 56, 106 in thermal insulating shell
70, 108 in firebox frame 80 and opening 110 in burner casing 34.
The positioning of the respective openings is such that a downward
sloping pathway is provided moving from rear to front. Openings
106, 108 and 110 receive one end of a fuel feed conduit 112 which
has its opposite end connected to a fuel feed system 113 described
below in more detail.
The front walls of firebox 56, thermal shell 70, firebox frame 80,
burner casing 34 and housing 12 each include a respective opening
114, 116, 118, 120 and 122 that allows exhaust conduit 28 to pass
therethrough. These openings are positioned so that exhaust conduit
28 is located at the end of the combustion gas pathway through the
firebox. In the illustrated embodiment, the bottom of the openings
are positioned at a level aligned with the bottom of the front wall
of the firebox 56. Positioning the openings at the bottom of the
front wall of the firebox 56 provides for the longest possible
pathway for the gases produced by the combustion of the pelletized
fuel. By maximizing the length of the pathway, the overall volume
of the firebox can be optimized.
The end of feed conduit 112 opposite firebox 56 is connected to a
fuel feed system 113. Fuel feed system 113 includes fuel conduit 26
having one end connected to a source of pellet fuel. Though not
illustrated, a suitable source includes a hopper that includes a
motor driven system for periodically delivering pellet fuel through
conduit 26 into housing 22. Pellet fuel exits conduit 26 inside of
housing 22 and is delivered into one end of a covered trough 126
that is rectangular in cross section. Referring to FIGS. 3 and 5, a
conventional helical auger 127 is provided in the forward end of
trough 126 for moving the deposited fuel towards the rear of the
trough. The rearward end of trough 126 includes an upward and
rearward extending ramp 128 over which the fuel is driven before it
drops down through the open bottom of the trough into a rotary vane
130 that serves as an airlock between the fuel source and firebox
56. Ramp 128 is provided to even out the flow rate of pellets
exiting auger 127. Auger 127 and rotary vane 130 are driven by a
single gearing mechanism 132 and a motor 134. The capacity of the
rotary vane is greater than the capacity of the auger and therefore
build-up of fuel within the rotary vane is avoided. An electrical
or mechanical sensor 124 is provided to detect the presence of
pellet fuel in supply tube 26 adjacent auger 127. When the fuel
level falls below a threshold setting, the sensor signals the fuel
source to deliver additional fuel. Beneath rotary vane 130, is
located the inlet of fuel feed conduit 112 which delivers the fuel
to grate 52. Fuel feed conduit 112 includes an opening 136 between
the point where it passes through burner casing 34 and passes
through firebox frame 80. Opening 136 permits combustion air within
manifold 142 (established between firebox frame 80 and burner
casing 34) to pass into the fuel feed conduit and be delivered into
the firebox as tertiary combustion air at a point above the burner
grate.
Housing 34 includes an opening for receiving combustion air from
the outlet of blower 140. Blower 140 provides combustion air into a
combustion air manifold 142 defined between burner casing 34 and
firebox frame 80, thermal insulating shell top 76 and firebox
support 46. Air from blower 140 passes through this manifold, down
around the sides of the firebox frame 80 and below firebox support
46. A portion of this combustion air enters the burn grate from
below and provides primary combustion air. A portion of this air
also enters the fuel feed conduit 112 through opening 136 and
provides tertiary combustion air above burn grate 52. Since firebox
56, thermal insulating shell 70 and firebox frame 80 do not provide
100% thermal insulation, thermal energy that passes through these
thermal barriers preferably serves to preheat the combustion air in
manifold 142. By preheating the combustion air, the efficiency by
which the wood pellet fuel is combusted is increased.
Located directly below firebox support 46 is ash pan 144. Ash pan
144 is supported above the floor of housing 12 by triangular
runners 146. Triangular runners 146 allow the ash pan to be slid
through an opening provided in burner casing 34 and out of housing
12 through door 30 for emptying.
In operation, the feeding of pellet fuel through conduit 26 is
controlled by the feedback from sensor 124 as described above.
Auger 127 is activated in response to a control system, such as a
thermostat. When it is desired to increase the temperature of the
exhaust gases, the gearing mechanism is activated so that auger 127
and rotary vane 130 cooperate to deliver additional fuel to the
combustion grate. Conversely, if a lower temperature for the
exhaust gases is desired, the control system delays activation of
the auger and the rotary vane to reduce the feed rate of fuel to
the combustion grate. In addition to adjusting the feed rate of
fuel, the control system also controls blower 140 so that
adjustments in the combustion air feed rate can be made.
The rotary vane 130 provides an airlock between the source of fuel
and combustion chamber, thus obviating the need to maintain a
positive pressure in the fuel vessel, and allowing blower 140 to
provide a positive pressure in the combustion chamber. Fuel that
exits rotary vane 130 falls through fuel feed conduit 112 and onto
burn grate 52, where it is combusted or can be ignited by igniter
94. Reciprocating arm 100 is activated periodically to break up
clinkers and remove ash from the rods. The removed ash falls into
ash pan 144 where it can be easily collected and removed. Gases
resulting from the combustion of the fuel move upward through the
serpentine path defined within the firebox 56 by baffle 68. The
thermal insulation provided to the combustion chamber of the
present invention allows for more complete combustion of the fuel
compared to pellet fuel burners which do not include a combustion
chamber that is substantially fully insulated. The more complete
combustion results because the temperature of the combustion
chamber is higher which encourages the more complete conversion of
the carbon monoxide and hydrocarbons to carbon dioxide and water
vapor as well as the burning of small particulate materials.
Exhaust carbon monoxide levels less than 200 parts per million, but
preferably less than 50 parts per million have been achieved by
burners formed in accordance with the present invention using wood
pellet fuel as the fuel source.
The thoroughness of the combustion achieved by the burner of the
present invention also allows for the design of a burner that is
compact and suitable for use in residential heating applications,
where space is often limited. As an example, burners formed in
accordance with the present invention having combustion chamber
volumes on the order of about 0.2 ft.sup.3 are able to output at
least 80K BTU/hr of thermal energy resulting in specific thermal
energy outputs of at least 400K BTU/hr/ft.sup.3, and preferably at
least 500K BTU/hr/ft.sup.3.
Another advantage of the compact nature of the present invention
relates to the relative response times that can be achieved by the
burner of the present invention. In space heating applications,
particularly precise control of the temperature of the exhaust
gases is imperative in order to provide the control necessary to
maintain the heated environment at the appropriate temperature.
Unlike larger incinerators or dryers, the relatively small volume
of the firebox in a burner of the present invention translates into
a relatively small thermal mass. With small thermal masses,
relatively quick changes in the temperature of the exhaust gases
can be achieved. For example, in a burner having combustion chamber
volumes on the order of 0.2 ft.sup.3, the temperature of the
exhaust gases can be varied at a rate of at least about 75.degree.
F. per minute, preferably 100.degree. F. per minute and most
preferably at least 125.degree. F. per minute.
As discussed above, the ability to maintain the gases produced by
the combustion of the pelletized fuel at a relatively high
temperature helps to incinerate particles that result from the
initial combustion of the pelletized fuel and therefore only small
amounts of solid particulate exit the firebox of the present
invention.
In order to avoid the build up of ash in the boiler, it is
preferred that as little ash exit the firebox as possible. Spaced
apart rods 92 and reciprocating rake 98 cooperate to minimize the
amount of ash exiting the firebox by processing the ash through
grate 52 and collecting it below grate 52.
While the preferred embodiment of the invention has been
illustrated and described, it will be appreciated that various
changes can be made therein without departing from the spirit and
scope of the invention.
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